1. Field of the Invention
The present invention relates to a fuel cell that vaporizes a liquid fuel stored in a fuel storage unit and uses the vaporized fuel gas to generate electric power.
2. Description of the Related Art
In recent years, together with the advance of portable type electronic equipment such as cellular phones, PDAs (Personal Digital Assistants), and notebook type personal computers, the batteries functioning as a driving power source and a memory retention power source are required to downsize, reduce weight, and increase capacity. For the current potable type electronic equipment, lithium ion batteries are used most commonly as the driving power source and so forth.
The lithium ion battery already exhibited a high driving voltage and a large capacity earlier when the battery was put into practice, and that, the performance of the battery has been improved to keep up with the advance of the portable electronic equipment. However, the lithium ion battery also has limitations in performance improvement, and is becoming impossible to satisfy requirements to serve as a driving power source or the like of the portable electronic equipment that will further improve to be highly functional from now on. On the back of this, developments were made for a new power generation device succeeding the lithium ion battery. As a result, a fuel cell promised to have a large capacity being several times of that of the lithium ion battery has finally been proposed.
Generally, the fuel cell is configured to include an anode (negative) and a cathode (positive) containing a catalyst, and a power generation unit composed of an electrolyte provided between the electrodes to allow certain traveling of ions. In the fuel cell, when a fuel and hydrogen are supplied to negative electrode and an air and oxygen are supplied to the positive electrode, an electrochemical reaction occurs at both the electrodes backed by the operation of the catalyst contained in the electrodes, so that direct current based on electron current can be brought out by the fuel as a supply source. In the fuel cell that generates power by such a mechanism, continuous power generation is possible for longer hours by replenishing the negative electrode with the fuel and the positive electrode with the oxygen, so that the fuel cell can be used in the same manner as a secondary battery and is expected applications to a power source for portable electronic equipment and the like.
The fuel cell is classified into a phosphoric-acid type, a proton exchange membrane type, a molten carbonate type, a solid oxide type, and so forth, in view of the type of its electrolyte. As a power source for the electronic equipment and the like, the fuel cell is required to be operative at a low temperature around a room temperature, downsized to compact, vibration tolerance, manufactured in large volume, and so on, so that the proton exchange type of fuel cell is considered to be appropriate.
In the proton exchange membrane fuel cell, as a fuel supply method, there are known methods of storing hydrogen gas to supply the hydrogen gas directly to the anode, storing organic fuel to supply hydrogen gas generated by reforming the organic fuel, storing a liquid fuel capable of supplying hydrogen to supply the liquid fuel directly to the anode, and so forth. Of these, the methods of supplying hydrogen directly and by reforming have difficulties such as in handling the hydrogen, in complicated configuration of equipment due to the reformation of fuel, and in indispensability of a device needing electric power, so that they are inappropriate as a small power source used in the portable electronic equipment. Accordingly, with a view to configure a small power source, the fuel cell adopting the method of supplying a liquid fuel, especially, a methanol solution to the anode, namely a direct methanol fuel cell system (DMFC) is gathering attentions.
Presently, many DMFCs have been developed. These DMFCs can be classified based on the fuel supply method to the power generation unit and the state of the fuel.
Firstly, based on the fuel supply system, the DMFCs can be classified into a system of compulsorily circulating the fuel by an auxiliary machine such as a pump, namely a so-called “active method”, and of supplying the fuel by natural diffusion such as of gravity and capillarity, namely a so-called “passive method”.
The DMFC of “active method” has a capability of mechanically controlling the concentration of the supplying liquid fuel, as a prime feature, in which water generated by a reaction when generating power is collected and then circulated. Therefore, problems of DMFC, a transmission of methanol to the electrolyte in the power generation unit, and accompanying performance degradation at the cathode, namely a crossover of methanol, can be prevented, and further, a methanol of higher concentration can be supplied.
Meanwhile, however, the DMFC of a “active method” has problems such as an increase in size due to its complicated mechanism, a need for electric power to operate, and so forth, being unsuitable to form the small power source for portable electronic equipment and the like. On the other hand, the DMFC of a “passive method” has a problem that the aforementioned crossover is difficult to be prevented therein even though it is easily downsized on the back of its simple mechanism.
Still further, in view of the state of fuel supplied to the power generation unit of the DMFC, the DMFCs can be classified into a “liquid supply system”, in which the liquid fuel is directly supplied to the power generation unit, and a “gas supply system”, in which the liquid fuel is vaporized and the fuel gas is supplied to the power generation unit.
In the DMFC of “liquid supply system”, it is possible to supply the liquid fuel to the power generation unit by a variety of techniques such as gravity, capillarity, natural diffusion, and so on, since the fuel is liquid. On top of that, the DMFC of “active method” can also transport the liquid fuel easily by pump or the like. However, for preventing the aforementioned crossover, the DMFCs are required to suppress the concentration of methanol in the liquid fuel to lower, approximately to 5% to 10%.
Meanwhile, in the DMFC of “gas supply system”, the concentration of the liquid fuel can be increased to higher since the fuel supplied to the power generation unit is a gas having smaller volume density than that of the liquid fuel, causing no concern about the crossover described above. In the case of the DMFC of “gas supply system”, when it is of “active method”, the mechanism is caused to be complicated, however, when it is of “passive method”, with its simple mechanism supported by a natural vaporization technique, the DFMC can supply a fuel gas of even high concentration directly to the power generation unit. As a DMFC of “gas supply system” as well as “passive method”, there is one disclosed in Japanese Patent No. 3413111 (Patent Document 1), as an example.